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Dichloromethane (DCM) or methylene chloride is the chemical compound with the formula CH₂Cl₂. It is a colorless, volatile liquid with a moderately sweet aroma. It is widely used as a solvent, the general view being that it is one of the less harmful of the chlorocarbons. Although it is not miscible with water, it will dissolve in most organic solvents. Approximately 500,000 tons were produced in 1995.

Dichloromethane was first prepared in 1840 by the French chemist Henri Victor Regnault, who isolated it from a mixture of chloromethane and chlorine that had been exposed to sunlight.

Production

Industrially, dichloromethane is produced by reacting either methyl chloride or methane with chlorine gas at 400–500 °C. At these temperatures, both methane and methyl chloride undergo a series of reactions producing progressively more chlorinated products.

CH₄ + Cl₂ → CH₃Cl + HCl

CH₃Cl + Cl₂ → CH₂Cl₂ + HCl

CH₂Cl₂ + Cl₂ → CHCl₃ + HCl

CHCl₃ + Cl₂ → CCl₄ + HCl

The output of these processes is a mixture of methyl chloride, dichloromethane, chloroform, and carbon tetrachloride. These compounds are separated by distillation.

Uses

Dichloromethane's volatility and ability to dissolve a wide range of organic compounds makes it an ideal solvent for many chemical processes. Concerns about its health effects have led to a search for alternatives in many of these applications.

It is widely used as a paint stripper and a degreaser. In the food industry, it is used to decaffeinate coffee and to prepare extracts of hops and other flavorings. Its volatility has led to its use as an aerosol spray propellant and as a blowing agent for polyurethane foams.

Speciallized uses

Dichloromethane chemically welds certain plastics (for example, it is used to seal the casing of electric meters). Often sold under the brand name of "EMA Plastic Weld" it is also used extensively in the modelmaking industry for joining plastic components together - it is commonly referred to as "Di-clo."

Its volatility is exploited in novelty items - drinking birds, bubble lights, and jukebox displays.

Toxicity

Dichloromethane is the least toxic of the simple chlorohydrocarbons, but it is not without its health risks as its high volatility makes it an acute inhalation hazard. Dichloromethane is also metabolised by the body to carbon monoxide potentially leading to carbon monoxide poisoning. Prolonged skin contact can result in the dichloromethane dissolving some of the fatty tissues in skin, resulting in skin irritation or chemical burns.

It may be carcinogenic, as it has been linked to cancer of the lungs, liver, and pancreas in laboratory animals. Dichloromethane crosses the placenta. Fetal toxicity in women who are exposed to it during pregnancy however has not been proven. In animal experiments it was fetotoxic at doses that were maternally toxic but no teratogenic effects were seen.

In many countries products containing dichloromethane must carry labels warning of its health risks.

See also

• Haloalkane
• Halomethane

References

External links

• National Pollutant Inventory - Dichloromethane Fact Sheet
• MSDS at Oxford University
• Dichloromethane at National Toxicology Program
• IARC Summaries & Evaluations Vol. 71 (1999)